There are many reasons why a verb may be lacking some forms. Most grammar treats defective verbs solely as those verbs where the forms are entirely impossible to use (that is, purely impersonal verbs such as pleuvoir (“to rain”) or grêler (“to hail”)) and verbs for which certain forms do not exist at all anymore, usually because of homonymy (choir (“archaic: to fall”), homonymous with choyer). This definition has generally excluded a number of verbs that, for example, have a conjugation that can be easily "completed", but, although not impersonal, are generally used only in the third person singular or plural in practice, as well as many reciprocal verbs that cannot be used in the singular.

Reasons for a verb to be truly defective are varied: the meaning of the verb may prevent its use in certain tenses, it might subsist solely as part of a set phrases or an idiom, or it might have been replaced in most uses by a more regular verb, leading to some forms falling out of uses. Finally, some adjectives that were once past participle of verbs with full conjugations are often used in compound tenses.

Additionally, some verbs are frequently given as defective in some tenses (most frequently the past historic and imperfect subjunctive), when in fact they are still used, although infrequently, in these forms.

The commonest of these verbs are often quoted with an infinitive in dictionaries, but are rare enough in modern use as to be functionally defective, as they are usually used most in compound forms. Those that descend from fully conjugated verbs may be considered merely archaic or obsolete.

This verb's infinitive remains only in the expressions faire accroire (“make to believe”) and the less frequent laisser accroire (“let believe”). The latter mostly has been replaced by the eggcornlaisser à croire, while the former is also undercut by faire à croire, criticised since the late nineteenth century, which is still considered a mistake. A related construction with savoir, inherited from Old French, has disappeared from use.

This verb, meaning "matter to" is fully impersonal and found in variation of the formal expression peu lui en chaut (“he couldn't care less”). The present is the most frequent form, but the infinitive, indicative imperfect (chalait), conditional (chaudrait) and present subjunctive (chaille) are also encountered.

This verb is a dated legal usage meaning "to appear before a court" and is used only in the infinitive and present participle. In all other tenses, comparaître (a reconstruction based on a more frequent verb) is used, and is also displacing it in these two forms.

This verb, meaning "to burn", mostly fell out of usage in the seventeenth and eighteenth century, leaving only its imperfect, subjunctive and infinitive with a modicum of liveliness. An equally literary and rare arder was reconstructed from these forms, and has a complete conjugation. A form ardoir also existed. The commoner words ardeur and ardant are related.

Like many verbs relating to animal cries (it means "to bray"), this verb, though it is often applied to people (meaning "to holler"), is primarily used in the third persons, where it has the conjugation of traire. However, the only tenses commonly encountered are the infinitive, indicative present, imperfect, future and conditional.

Although it may be most common in the third persons, the other forms of those tense that are in uses are not unknown. Its most well-known use in the future (which is otherwise exceptional) is in the traditional Charles Perrault version of Little Red Riding Hood: "Tire la chevillette, [et] la bobinette cherra." The major tenses are the past participle, infinitive, indicative present and past historic. One could presumably easily reconstruct missing forms from the more complete déchoir or échoir (although strictly speaking, the verb is probably conjugated like asseoir, hence the variation in the future tense), but this is not usually done.

This verb is never used outside the third person, and has no imperative. It also has several alternative forms. It is frequently given as missing the indicative and subjunctive imperfect, but this appears to be an oversimplification based on the fact the tenses are missing from choir.

Although the conjugation of this verb could conceivably be aligned on that of rire, it has long been supplanted in most of its conjugation by a causative periphrasis: faire frire since it is an unaccusative verb. The only tenses encountered are the singular present indicative, the future, conditional, and, much less commonly, the compound tenses.

Meaning "to be inappropriate", this verb has the conjugation of seoir (see below), but is mostly restricted to the indicative present third person singular(il messied), with other forms used much less commonly.

This archaic verb is mostly restricted to the infinitive and past participle (occis), but its conjugation can be readily completed with that of circoncire, and some of these forms have been used by respected authors.

This verb, meaning "hear, listen" is very rare outside the infinitive and compound tenses. In modern use, there is a tendency to reconstruct other forms as a second-group verb. Its second-person imperative oyez, used in public announcement in historic fiction, is probably the best-known form. The original, obsolete conjugation is very rarely resurrect by some, and is given below:

This verb is conjugated like connaître, but unlike croître, is not used in those tenses where it is homographic with pouvoir, namely the compound tenses and past participle, past historic, and subjunctive imperfect.

This is a rare example of a verb whose infinitive is one of the forms that are almost never used. It is used only in the third persons and mostly in the meaning "to be appropriate". Its participles sis (“located”), séant (“law: sitting”) and seyant (“fitting”) have taken distinct meanings of their own.